The present application relates to battery packs of the type usable with cordless power tools and, in particular, to lead-acid battery packs. The application relates in particular to techniques for protecting against deep discharge of lead-acid battery packs.
Cordless power tools commonly use a NiCad battery pack to power an electric motor. Such tools are commonly used until the battery pack is completely discharged, since NiCad cells can tolerate deep discharges and, in some circumstances, are actually revived by deep discharge/charge cycling. Furthermore, the discharge of a NiCad battery is apparent to the user, since, as the battery approaches deep discharge, the power deliverable by the tool is reduced until, eventually, the tool will stop working.
It is also known to utilize lead-acid batteries in various devices because such batteries afford certain power delivery advantages. Such lead-acid batteries, of a size suitable for use in cordless power tool battery packs, are sold by Bolder Technologies under the designation "Bolder 9/5 sub-C TMF" (Thin Metal Film). However, unlike NiCad batteries, lead-acid batteries do not tolerate deep discharges. Discharge of lead-acid batteries below about 50% state-of-charge adversely affects battery life and could cause permanent damage. Thus, to ensure maximum battery life, such batteries should not be discharged below about 50% state-of-charge. Furthermore, when such lead-acid batteries are utilized in battery packs for cordless power tools, the discharge of the battery to about 50% state-of-charge may not be readily apparent to the user, since the battery can still deliver substantial power at such state-of-charge levels. Thus, the user may easily discharge the battery to below 50% state-of-charge without knowing that he is doing so.